1. Field of the Invention
The present invention relates to a surface acoustic wave device that is used, for example, as a resonator or a band-pass filter and, more particularly, to a surface acoustic wave device that includes an IDT electrode and a silicon oxide film provided on an LiNbO3 substrate and that utilizes an SH wave.
2. Description of the Related Art
A band-pass filter used in an RF stage of a cellular phone, or other suitable device, must have a wide band and desirable temperature characteristic. Thus, the related art utilizes a surface acoustic wave device such that an IDT electrode is provided on a piezoelectric substrate made of a Y-rotated X-propagating LiTaO3 substrate or a Y-rotated X propagating LiNbO3 substrate and a silicon oxide film is arranged so as to cover the IDT electrode. The piezoelectric substrate of this type has a negative frequency-temperature coefficient. Thus, in order to improve the temperature characteristic, a silicon oxide film having a positive frequency-temperature characteristic is arranged so as to cover the IDT electrode.
However, with the structure described above, when the IDT electrode is made of Al or an alloy that primarily includes Al, which are typically used, the IDT electrode has not been able to attain a sufficient reflection coefficient. Thus, there has been a problem that a ripple tends to occur in the resonance characteristic.
To solve the above-described problem, WO2005-034347 discloses a surface acoustic wave device including an IDT electrode made of a metal having a density that is greater than that of Al that is provided on a piezoelectric substrate made of LiNbO3 having an electromechanical coefficient K2 of at least about 0.025, a first silicon oxide film is provided in the area, other than the area in which the IDT electrode is provided, so as to have a thickness substantially equal to the electrode, and a second silicon oxide film is laminated so as to cover the electrode and the first silicon oxide film.
In the surface acoustic wave device described in WO2005-034347, the density of the IDT electrode is greater than or equal to about one and half times the density of the first silicon oxide film, and, therefore, the reflection coefficient of the IDT electrode is sufficiently increased to thereby suppress a ripple that occurs in the resonance characteristic.
In addition, in WO2005-034347, a Rayleigh wave is utilized and Au, Cu, or other suitable material, is used as the electrode material. The structure in which the electrode made of Cu has a thickness of about 0.0058λ to about 0.11λ is described. In this case, the LiNbO3 substrate having Euler angles of (0°±5°, 62° to 167°, 0°±10°), and preferably, (0°±5°, 88° to 117°, 0°±10°), is described, and the thickness of the second silicon oxide film and ranges from about 0.15λ to about 0.4λ where λ is the wavelength of a surface wave.
In the surface acoustic wave device described in WO2005-034347, when θ of the Euler angles of the LiNbO3 substrate, the thickness of the electrode made of Cu and the thickness of the second silicon oxide film are set so as to fall within the specific ranges described above, it is possible to increase the electromechanical coefficient of a mode to be used, and it is possible to reduce the electromechanical coefficient of a mode that becomes a spurious response.
That is, WO2005-034347 describes that it is only necessary that the Euler angles of the LiNbO3 substrate are set so as to reduce the electromechanical coefficient of the mode that becomes a spurious response.
However, for example, when a surface acoustic wave filter device is used as a duplexer of a cellular phone, the transmission-side filter is required to increase attenuation in the pass band of the reception-side filter, which is the filter at the other side. Thus, in order to increase attenuation in the pass band at the other side, a method of varying a duty of the IDT electrode is known.
That is, usually, the duty of the IDT electrode is set to about 0.5. However, there is a case in which the duty is increased to greater than about 0.5 in order to increase attenuation in the specific frequency band. In the surface acoustic wave device described in WO2005-034347, the Euler angles of the LiNbO3 substrate, which reduce the electromechanical coefficient of the mode that becomes a spurious response is described. However, when the duty of the IDT electrode is varied as described above, even when the Euler angles described in WO2005-034347 are selected, a spurious response may still be too large. That is, in the surface acoustic wave device described in WO2005-034347, the duty of the IDT electrode has not been considered.
In addition, depending on the conditions under which the surface acoustic wave device is manufactured, a spurious response has occurred due to variations in the size of each electrode finger in the width direction.